Metal pickling solutions and methods



United States Patent 2,876,144 METAL PICKLING SOLUTIONS AND METHODS No Drawing. Application February 24, 1956 Serial N0. 567,453

9 Claims. (Cl. 134-3 This invention relates to improvements in pickling solutions and in methods of picklin metals to remove surface scale, oxides and other jnriaeejeritaj ifiifitir.

More particularly, the invention relates to pickling metals l ke titanium and zirconium and their alloys containing titanium or zirconium as major constituents, which are subject to surface embrittlement due to absorption of oxygen, nitrogen, hydrogen, etc., when heated in air at elevated temperatures such as those required in heat treating or for hot rolling or forging operations or the like. Removal of this embrittled surface layer improves ductility, particularly bend ductility.

Pickling baths or solutions for alloy steels are generally acidic in nature, but most acids commonly employed for the ferrous alloys do not readily attack corrosion resistant metals like titanium, zirconium and their alloys. Among those acids which do, a majority hydrogen-embrittle and/or deeply pit the surface of the metal. Acid pickling solutions which contain appropriate concentrations of fluoride ions constitute important exceptions to the foregoing general observation. Aqueous solutions containing about by weight nitric acid and about 2% hydrofluoric acid have been employed as pickling baths for alloy steels, and these solutions are eifective to a 1imited extent in removing surface scale for titanium, for example. However, they are subject to the following serious limitations: (1) A substantial amount of hydrogen is absorbed by the metal resulting in serious loss of ductility; (2) copious quantities of nitric oxide are produced which react with oxygen of the air to form hazr ardous nitrogen dioxide in quantities sometimes exceedlng available ventilating capacity; and (3) the bath is difficult to control. In co-pending U. S. patent application Serial Number :i 4 3 5 755, filed JunelO, 195 of which the present applicationisabohtinuation in-part, there is described in detail the basic electro-chemical phenomena which take place during pickling of titanium in such nitric acid-fluoride ion .solutions. Therein it is pointed out that there is a critical nitric acid concentration for these fluoride ion-containing pickling solutions, namely, above by weight HNO When the nitric acid concentration is be,- low 15%, substantial amounts of hydrogen are absorbed by the titanium. The aforementioned application provides an aqueous pickling solution containing from about 15 to 50% nitric acid and about 0.5 to 10% of fluoride ion. At these concentrations the pickling solution gives an acceptable finished surface with a minimum of hydrogen absorption. A solution containing about 30% nitric acid and 4% hydrofluoric acid produced exceedingly good results, dissolving about 0.001" of titanium-7% manganese sheet in a minute at 150 F., with very little hydrogen evolution and absorption by the metal. There err-use iitii'tttanbt y 0.

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H1 L M HNO 5 2,876,144 lCe Patented Mar. 3, 1959 is no significant reduction in hydrogen pick-up by the metal at concentrations above 30% HNO and increase of nitric acid beyond about 50% by weight of the solution is not economical.

The only objectionable feature to the nitric acid-flu oride ion pickling solution of the co-pending application is the evolution of poisonous 1119 535. This disadvantage may be overcome 5 the additigw Qe pick1ing glun f fisqt 9.1 .72. .brqrcssane ss The arbitrate-arrears itself an excellent oxidizing agent and is more easily reduced than the hydrogen ions formed during pickling. Accordingly, when pickling titanium in a nitric acid solution containing fluoride ions and hydrogen peroxide, the peroxide is reduced to water along with the formation of titanium tetrafiuoride, and substantially no hydrogen is evolved which could be absorbed by the metal- Since h seuemxideiaitaelfetfective in 1; bination -vTith uori cle ions as a pickling solution, when empldyifwilh the nitric aciZFfluoride ioii solution the lfilfiftfi ,nit'risae sm iibsz ie u In the C04 pending application, an a ueous solution containing from 2 to 50% HNO 0.5 to 30% H 0 and 0.5 to 10% fluoride 10118 was found to be quite eflfective on the aforementioned metal surface. Solutions of this type give excellent finished surfaces, do not fume, and are the most economical. Optimum results were obtained with such solutions containing about 5% HNO about 2% H 0, and about 2% fluoride ions.

Also in the aforementioned application we described a pickling bath for the above metals containing from 0.5 to 5% fluoride ions and from 5 to 35% hydrogen peroxide, i. e., a solution which does not contain nitric acid. This solution has the advantage of no poisonous or otherwisenbjectional gas evolution; it imparts a good surface finish to the metal treated, and results in very little hydrogen pick-up. The products of the reaction of this solution with the metal treated are simply water and a titanium or other metal complex. Optimum results were obtained with an aqueous bath containing about 20% H 0 and about 2% fluoride ions. At F. this bath pickled oif about 0.001" of metal per minute.

Each of the three types of pickling solutions disclosed in our co-pending application are eminently satisfactory for removing surface scales produced by heat treatment or hot working of titanium, zirconium and their alloys below about 1000 F. However, the aforementioned pickling solutions are not effective for removing scales from metals hot worked or stress-relieved at 1000 F. or higher. Non-uniform pickling occurs, which results in a very rough finish, particularly if the metal surface has been marred by dirt, oil, ink markings, fingerprints, etc., prior to heat treating. Forming and stress-relieving titanium, zirconium and their alloys at temperatures around 1000-1200 F. is often advantageous in some operations, and accordingly, a pickling solution which will satisfactorily remove the diflicult and troublesome films produced on the surface at these temperatures would be highly desirable.

We have discovered that the addition of phosphoric acid to the pickling solutions of U. S. patent application Ser. No. 435,,755 produces a pickling bath which is highly effective for removing surface scales from titanium, zirconium and their alloys formed at or above 1000 F. The phosphoric acid modified solutions produce a white, somewhat etched surface instead of the usual lustrous finish. This flat etched finish is often advantageous, espe cially when a high luster is undesirable, as in finishing belt-ground sheet, to minimize the appearance of strikes.

The present invention contemplates aqueous solutions containing in percentages by weight about 0.5 to 10% of fluoride ions, i. e., hydrofluoric acid or equivalent amounts of fiiiSFideEaIts, such as NaF, NaBF NH.,F, etc., about 0.5 to 10% phosphoric acid, and an oxiiiziggagent sele'ctedfronfohebfthe'followingz about 15 to 50% nitric acid, about 5 to 35% hydrogen peroxide, and about 2 to 40% nitric acid together with about 0.5 to 30% hydrogen peroxide. In most instances, the phosphoric acid concentration will be between 1% and 6% by weight of the solution, with 2% to 5% H PO being preferred. A pickling solution which is particularly effective for removing the difiicult light scales formed at l000-1100 F. is one containing about 20% HNO about 3% HF and about 2% H PO Pickling is carried out in accordance with the present method at temperatures between 100 and 190 F.

A senes of tests were conducted to establish that the improved effectiveness of the pickling solutions in removing high temperature scales was definitely attributable to the presence of phosphoric acid, as distinguished from wetting agents often employed in pickling baths, and more particularly, other mineral acids. A sheet of titanium metal was halved and one half scaled in air at 1000 F. for 30 minutes, and the other half scaled at 1100 F. for a like period. Each of the sample sheets was then pickled in a 20% HNO 3% HF bath maintained at l60-170 F. This solution was used as a standard for comparison. The standard solution did not remove the 1000" F. scale in a satisfactory manner, and gave a rough finished surface. Its action on the 1100 F. scale was considerably less effective, producing a very rough surface. Variations in bath temperature did not have a significant effect upon the ability of the standard solution to remove these oxidized films, nor did any changes in the HF and HNO, concentrations of the bath produce beneficial results. The standard solution was then modified by the addition of 0.1% by weight of Quaker States wetting agent Queen No. 5. No improvement in pickling was observed on either the 1000 F. or 1100 F. films. Addition to the standard solution of 0.1% by weight of Haas Miller Corporations surface active agent Actival 57 likewise did not improve removal of the films. A further series of tests was run with the standard solutions modified by the addition of 5% H 80 10% H 80 and 5% HCl respectively. There was no apparent improvement in surface characteristics of the sheets pickled in these solutions over those produced by the standard solution.

Phosphoric acid was then added to the standard solution in concentrations of 2%, 5% and 10% H PO Unexpectedly, the phosphoric acid-containing solutions indicated a definite improvement. Each of the phosphoric acid-containing baths completely removed the 1000 F. scale and entirely satisfactorily attacked the 1100 F. scale on the sample sheets, and in all cases produced a smooth, white, etched surface. Subsequent tests indicated that the addition of only 0.5% H PO to the standard solution produced significant improvements in high temperature scale removal. We observed that for the above standard solution the optimum phosphoric acid concentration appears to be about 2% by weight.

Phosphoric acid additions to H O HF and HNO -H O -HF pickling baths also enable these solutions to satisfactorily remove similar high temperature-produced surface scales. If desired, the improved pickling solutions may be modified by the addition of suitable wetting agents, such as the two mentioned above, which minimize vertical striations produced by nonuniform pickling due to the rapid vertical movement of gas bubbles and solution along the sheet surface.

What is claimed is:

1. An aqueous solution for pickling titanium, zirconium and alloys of each with removal of basis metal from the surface without attendant hydrogen embrittlement, comprising in percentages by weight, about: 0.5 to 10% phosphoric acid, 0.5 to 10% of fluoride ions, and at least'o'ri member selected from the group consisting of 15 to 50% nitric acid, 5 to 35% hydrogen peroxide, and 2 to 40% nitric acid together with 0.5 to 30% hydrogen peroxide.

2. An aqueous solution for pickling titanium, zirconium and alloys of each as set forth in claim 1 comprising in percentages by weight, about 0.5 to 10% of fluoride ions, about 15 to 50% nitric acid and about 0.5 to 10% phosphoric acid.

3. An aqueous solution for pickling titanium, zirconium and alloys of each as set forth in claim 1 comprising in percentages by weight about 0.5 to 5% of fluoride ions, about 5 to 35% hydrogen peroxide and about 0.5 to 10% phosphoric acid.

4. An aqueouTs dlil'fiofiforpickling titanium, zirconium and alloys of each as set forth in claim 1 comprising in percentages by weight about 0.5 to 10% of fluoride ions, about 2 to 40% nitric acid, about 0.5 to 30% hydrogen peroxide and about 0.5 to 10% p h osphoricaacid 5. The method of pickling titanium, zirconium and alloys of each with removal of basis metal from the surface without attendant hydrogen embrittlement, which comprises immersing the metal in an aqueous pickling solution containing in percentages by weight about 0.5 to 10% phosphoric acid, about 0.5 to 10% of fluoride ions, and afieasfo'fierhe'mber of the group consisting of about 15 to 50% nitric acid, about 5 to 35% hydrogen peroxide, and about 2 to 40% nitric acid together with 0.5 to 30% hydrogen peroxide, while maintaining said solution at a temperature of about -190 F.

6. The method of pickling titanium, zirconium and alloys of each as set forth in claim 5, which comprises immersing the metal in an aqueous pickling solution containing in percentages by weight about 0.5 to 10% of fluoride ions, about 15 to 50% nitric acid and about 0.5 to 10% phosphoric acid while maintaining said solution at a tempemout 100-190 F.

7. The method of pickling titanium, zirconium and alloys of each as set forth in claim 5, which comprises immersing the metal in an aqueous pickling solution containing in percentages by weight about 0.5 to 5% of fluoride ions, about 15 to 35% hydrogen peroxide and about 0.5 to 10% while maintaining said solution at a temperature of about 100-190 F.

8. The method of pickling titanium, zirconium and alloys of each as set forth in claim 5, which comprises immersing the metal in an aqueous pickling solution containing in percentages by weight about 0.5 to 10% of fluoride ions, about 2 to 40% nitric acid, about 0.5 to 30% hydrogen peroxide and about 0.5 to 10% phosphoric acid v hile maintaining said solution at a tem- ITeFafifrFof about 100-190 F.

9. The method of pickling titanium and its alloys with removal of basis metal from the surface without attendant hydrogen embrittlement, which comprises immersing the metal in an aqueous pickling solution containing in percentages by weight about 1 to 5% of fluoride ions, about 20 to 35% nitric acid and about 1 to 6%, phosphoric acid while maintaining said solution at a temperatureab'o'ut 100-190 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,974,570 Kiefer Sept. 25, 1934 2,564,749 Bried Aug. 21, 1951 2,598,889 Caugherty June 3, 1952 (Other references on following page) 5 UNITED STATES PATENTS 6 FOREIGN PATENTS Norway Oct. 17, 1949 OTHER REFERENCES Durkin: Metallurgia, vol. 49, page 256, May 1954 (from October 1953 article).

Handbook on Titanium Metal, 7th ed., pages 40-44, 49, 88, 89, 93, pub. by Titanium Metals Corp. of America, New York (August 1, 1953). 

1. AN AQUEOUS SOLUTION FOR PICKLING TITANIUM, ZIRCONIUM AND ALLOYS OF EACH WITH REMOVAL OF BASIS METAL FROM THE SURFACE WITHOUT ATTENDANT HYDROGEN EMBRITTLEMENT, COMPRISING IN PERCENTAGES BY WEIGHT ABOUT: 0.5 TO 10% PHOSPHORIC ACID, 0.5 TO 10% OF FLUORIDE IONS, AND AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF 15 TO 50% NITRIC ACID, 5 TO 35% HYDROGEN PEROXIDE, AND 2 TO 40% NITRIC ACID TOGETHER WITH 0.5 TO 30% HYDROGEN PEROXIDE. 